1. Field of the invention
This invention relates to a method and apparatus for processing cables to achieve desired physical characteristics, in particular reduced longitudinal stretch.
2. The prior art
In many applications, it is important that the physical length of a cable be fixed or if the length changes, such changes be determinable. This is significant, for example, in well logging where a sonde is lowered into a well on an electric cable and the depth of the sonde is determined by measuring the amount of cable payed out. Usually, the length of cable in such an application changes with temperature and tension on cable, but these length changes can be calculated from known temperature and tension data relationships. However, if the cable is irreversibly deformed in use (as by a permanent or plastic elongation), the cable length has changed and the precise depth of a logging sonde in the well at the time of use is not accurately determined. Moreover, if the cable length is determined by markers along the length of the cable, these markers are made inaccurate by permanent cable elongation.
It has been proposed, in order to solve the problem of irreversible deformation of a cable, to subject the entire length of the cable before use to a "hot pre-stressing" operation. In this operation, the cable is subjected to simultaneous tension and heat in such a manner that the inherent irreversible (or plastic) deformation characteristics are substantially removed; the only deformation characteristics which are supposed to remain at that point are the elastic deformation characteristics.
Nevertheless, this operation does not fully overcome the above mentioned difficulties.
The kind of cables the invention is directed to generally comprises a core surrounded by a jacket, made e.g. of insulating material. The jacket is itself enclosed by a first and a second layer of armor strands. The core may include electrical conductors and/or optic fibers and the usual electrical insulating and mechanical protecting sheaths immediately surrounding the electrical conductors or the optic fibers. Such known cables are described e.g. in A. Blanchard U.S. Pat. No. 2,725,713; Nance et al. U.S. Pat. No. 3,106,815; W.E. Bowers et al. U.S. Pat. No. 3,137,988; and Whitfill, Jr. et al. U.S. Pat. No. 3,800,066. All these patents are assigned to predecessors in right or subsidiary of the assignee of the present application, and are incorporated herein by reference. In a first embodiment, as shown in the article "Umbilical Cable Design and Selection Criteria" from Fred Hartdegen and Willem Wijnberg, published in Ocean Industry, Mar. 1985, May 1985 and Jun. 1985, the jacket disposed between the core and the armor strands is made of a thermoplastic material, such as e.g. Polyethylene or Ethylene Propylene Copolymer (EPC). This thermoplastic material is such that it allows, upon tension applied to the cable during use, the armor strands to embed into the jacket material. Especially, the armor strands lie in grooves generated on the periphery of the jacket. The grooves, which remain after release of the tension, help to maintain the armor strands in a close relationship with the jacket/core. However, upon tension applied to the cable, the jacket thermoplastic material is usually squeezed and, after having filled in the interstices between the armor strands, may even ooze out between the armor strands. As a consequence, the armor strands embed in the jacket deeper and deeper from one utilization to the next one, and thus the cable becomes longer and longer, in a permanent way. This plastic elongation of the cable goes on until the armor strands are in contact one with another. At that point, the wear of the cable increases rapidly.
One has suggested to remedy this situation by making the jacket out of a thermosetting material, such as a cured rubber e.g. acrylonitrile butadiene rubber, as shown in the above referred to patents. At rest, the armor strands lie on the cylindrical surface of the jacket. Upon application of tension on the cable, the cured rubber softness allows the armor strands to embed in grooves generated at the surface of the jacket, while its elastic properties make the embedding grooves disappear upon release of the tension.
The rubber jacket cable, although it seems to obviate the oozing phenomenon above referred to, shows some drawbacks.
First, the rubber jacket cable usually stretches longitudinally, when subjected to tension, to a bigger amount than the plastic jacket cable, since the rubber fills in the interstices between the armor strands, thus allowing the cable to stretch. Second, the rubber is a material which is not easy to handle and to process, and which also presents some limitations with respect to temperature resistance and aging.
Accordingly, there is a need, especially in a hostile environment such as in a borehole, for a cable showing no permanent (plastic) stretch and only limited elastic stretch.